This application claims Paris Convention priority of DE 101 04 365.1 filed Feb. 1, 2001 the complete disclosure of which is hereby incorporated by reference.
The invention concerns a superconducting magnet system, in particular for a high-resolution magnetic resonance spectrometer, comprising a substantially cylindrical cryostat with an axial room temperature bore for receiving a sample, a radio frequency transmitting and detecting system, and a main coil which is superconductingly short-circuited during operation and is located in a first, radially inner region within the cryostat at a first low temperature level, and surrounds the sample in the room temperature bore and generates, during operation, a homogeneous temporally stable magnetic field at the sample location which satisfies the requirements for recording a high-resolution magnetic resonance spectrum.
Means for stabilizing the magnetic field generated by a superconductingly short-circuited main coil located in a cryostat in the measuring volume of a high-resolution magnetic resonance spectrometer, in particular a high-resolution NMR spectrometer, are known from superconducting magnet systems of NMR spectrometers built by the assignee, wherein the means comprise one or more compensation coils which are dimensioned and located such that, in their entirety, they are suitable for largely compensating field drifts of the superconductingly short-circuited main coil in the measuring volume.
In addition to an extremely good magnetic field homogeneity over the sample volume, high-resolution NMR spectrometers must also have a likewise good temporal stability for the magnetic field. Towards this end, the superconducting main coil of the magnet is superconductingly short-circuited during operation. The requirements for the properties of the superconducting short-circuit switch, the quality of the superconducting coil wires, and for the superconducting joints between the individual wire sections of the coil are therefore stringent. Overall decay times for the superconducting coil current of tens of thousands of years must be guaranteed during short-circuit operation.
Temporal fluctuations of the magnetic field at the sample location can be compensated for by a so-called lock system. Towards this end, the spectrometer measures a separate NMR signal of a lock substance (i.a. deuterium) usually within an associated frequency band and its frequency is stabilized via a feedback circuit by means of a small resistive compensation coil (lock coil) in the room temperature bore of the magnet system.
A superconductingly short-circuited magnet coil keeps the magnetic flux through its bore constant, i.e. the superconducting current changes spontaneously e.g. in response to an external disturbance field such that the total flux through the coil does not change. This usually causes the field in the working volume to become inhomogeneous and to change since the spatial distribution of a disturbance field and that of the main magnet coil are different. Compensation for these deviations through the geometrical design of the main coil, using additional superconducting coils or active control measures has been proposed (V. V. Sukhol et al., Voprosy Atomnoi Nauki I Techniki, Seria Obshchaya i Yademaya Fizika 1981, Vol. 3 (17) 1-81, U.S. Pat. Nos. 4,974,113, 4,788,502, 5,278,503).
The superconducting magnets of high-resolution NMR generally use superconducting shim coil sets in a first step towards homogenizing the field at the sample location. During operation, a correction current is applied to the individual coil sets and superconductingly short-circuited. The shim coil sets can also comprise a so-called B0 coil which can generate a small sufficiently homogeneous additional field at the sample location. Such a coil permits precise setting of the field, or the proton frequency, to a predetermined value without interrupting the superconducting current circuit of the main coil. Moreover, a drift in the main coil can also be compensated for, within given limits, via the short-circuited B0 coil. Towards this end, the B0 coil must be positioned and dimensioned such that the field decay of the main coil induces a counter current in the B0 coil which causes the field at the sample location to remain constant. This method is limited in that the current through the B0 coil must not exceed a certain value due to e.g. the wire used. In any case, the contribution of the (inhomogeneous) B0 coil must remain sufficiently small that the field homogeneity over the sample is not impaired. Moreover, the required inductive coupling of the B0 coil to the main coil during a quench can cause excessive current flow and destruction of the B0 coil which requires protective counter measures and associated additional expense.
Although the production of superconducting high field magnets for high-resolution NMR spectrometers (or also ICR spectrometers) has reached a very high level of quality and reliability, some of the very expensive magnet systems nevertheless clearly exceed the specified drift limits while otherwise being quite stable. Attempted compensation of drift using the lock coil or a B0 coil of the shim system would quickly lead to the above-mentioned limits and the intervals between post-regulation of the total field (including associated opening of the superconducting main current circuit, introduction of current rods, helium loss etc.) would be intolerably short.
For this reason, there is a need for a superconducting magnet system of the above-mentioned type which can compensate for drifts of a magnitude above the maximum specified limit, over long periods without inadmissibly impairing the homogeneity and stability of the magnetic field at the sample location. In particular, the use of existing drifting main coils should be possible.
This object is achieved in that at least one superconducting drift compensation coil is located in a second radially outer region within the cryostat at a second higher temperature level within the cryostat.
A superconducting compensation coil, in particular of high-temperature superconducting material can be disposed at a temperature level above the main coil, radially outside of the main coil, in particular in a nitrogen tank of the magnetic cryostat or in thermal contact with the refrigerator stage of the cryostat in the temperature region between 20K and 100K. A compensating current flows through the compensation coil and compensates for the decay of the magnetic field due to the drift of the main coil at the sample location. Maintenance of the compensation field with sufficient homogeneity using e.g. an appropriate Helmholtz arrangement is easier at larger separations from the sample location.
The use of a superconducting wire ensures generation of a sufficiently large current. Arrangement outside of the main coil, in particular in the nitrogen tank, ensures that the superconducting main coil must not be altered and modifications of the helium tank installation are not necessary.
In a preferred embodiment, the compensation coil is connected to an external current source and the compensation current is regulated, or merely controlled, through measurement of the field at or close to the sample location. Active regulation avoids generation of an additional superconductingly short-circuited current circuit which could charge in an uncontrolled fashion. When the current source is switched off, the compensation current stops flowing. On the other hand, the use of a superconductor reduces the heat input e.g. into the nitrogen tank to a negligible level. Regulation preferably occurs through a lock circuit, optionally in addition to and as a supplement to the already existing resistive lock. A coarse compensation might be sufficient, optionally in steps of long duration. The resistive lock system thereby performs the fine adjustment. The fact that the contribution of the compensating field to the overall field is very small (up to approximately 10xe2x88x925) permits toleration of inevitable noise even in the high-resolution spectrometer since it can be easily maintained below 10xe2x88x926. Of course, it should be noted that inductive couplings of the compensation coil to the superconducting circuits of the main coil and to the shim system are either eliminated in the arrangement or taken into consideration in the design.
In an alternative embodiment, the compensation coil comprises a superconducting switch and is at least temporally superconductingly short-circuited during operation. Superconducting wires of HTS materiel do not currently have the low ohmic resistance required for NMR applications, in particular, for large currents. Recently, however, there has been promising progress which anticipates HTS sections becoming part of a main coil, even at full current. It seems feasible that they may soon be used in a suitable superconductingly short-circuited compensation coil of low current. But even a compensation coil with some drift can still reduce the drift of a main coil for a relatively long period of time as long as the compensation field produced by It has a smaller drift (absolute not relative!) than that of the main field. Optionally, even two or more nested compensation coils could be used, with a decay of the current in the inner coil inducing a larger compensation current In the outer coil just as a current decay of the main coil produces a current in the inner compensation coil which is larger than the current decay of the main coil. The compensation current (or currents) can be reset from time to time to their nominal value by an external regulatable or controllable power supply.
With the exception of the highest field strengths, the magnet systems of modern NMR or ICR spectrometers are usually actively shielded, i.e. the main coil consists of two partial coils which are connected in series and produce opposite fields such that the total dipole moment of the arrangement and therefore the stray field substantially vanishes. Moreover, as mentioned above, resistive and/or superconducting compensation arrangements are usually provided which minimize the influence of external disturbances at the sample location. The present invention is preferably used in combination with these measures, wherein it should always be noted that the various shielding and compensation means interact. This must either be partially eliminated (zero coupling) or the overall behavior of the apparatus must be explicitly taken into consideration in the design.